ONLINE MEDIA

TEACHER'S GUIDE

1. This activity follows the same format as the previous food web and worksheet. Working in the same groups of two, students use the new worksheet to analyze the food web of today’s Chesapeake Bay with large-scale fishing (worksheet 2).

a. This food web is much less complex (and easier to read) than the food web that the students previously analyzed. Some species are missing including alligators and manatees (Imagine that they used to live in the Chesapeake Bay!), but others are now rare like whales, sharks, seals, predatory fish, grazing fish, predatory invertebrates, oysters, seafloor algae and plants, sea grass, and sea turtles. (Imagine that they used to be abundant!)

b. Tell students that sea turtles used to be so abundant in the Caribbean that during the second voyage of Columbus a sailor described “that it seemed as if the ships would run aground on them, and their shells actually clattered" along the topsides. Scientists estimate that there were over 30 million sea turtles in the Caribbean alone before people started to harvest them. (http://www.flmnh.ufl.edu/sciencestories/2008/mother_sea_turtle.htm). Many feeding relationships that were once strong are now weak. Students are invited to think about why the food web is so different and how the changed food web might impact ecosystem health.

2. The worksheet moves the students through many important issues including asking students why jellyfish are abundant if historically they were rare and asking students why predatory fish are rare, even though their shark, seal, and whale predators are now rare.

a. The worksheet then moves to the problem of pollution in the Bay. Students are asked to use what they know about eutrophication to explain why the algae are now abundant in the Bay. This is also a good point in the lesson to link the growth of algae to farming and the loss of filtering organisms, as the plankton (floating algae) levels start to rise in the mid-1700s with the introduction of farming, but hold steady while the oysters are still present in the Bay.

b. The floating algae then rapidly spike after oyster harvesting removes most oysters from the Bay (this will be covered in more detail in worksheet 3). As a side note, they are also asked how the abundance of floating algae might affect the abundance of plants that grow below them like seafloor algae, plants and sea grass.

3. These questions begin to explain the state of the Bay today. Nutrients added to the Bay feed the growth of floating algae. The abundance of algae floating on the surface block the sunlight from reaching sea floor algae and plants, causing them to become rare.

a. Finally, students are asked to connect the high levels of nutrients, floating algae, and microbes to the loss of the oyster filters in the Bay. They develop a hypothesis about how the loss of oyster filters might contribute to the high floating algae levels in the Bay (If people over-harvested the oysters, then floating algae would increase).

b. After students develop their hypothesis and determine the type of data they would need to analyze to test their hypothesis, give students data on current and historic levels of oysters and algae in the Bay (worksheet 3).